05 Audi A-6 3.2l. overheating. No pressure on coolant hose. Why?

Begin with the basics....
INSPECT COOLANT LEVEL,
Low ????,PRESSURE TEST SYSTEM,
PRESSURE TEST RADIATOR/EXPANSION TANK CAP,
INSPECT DRIVE BELT(s),
INSPECT COOLANT HOSES,
INSPECT HEATER HOSES,
INSPECT BYPASS/MISC. COOLANT HOSES/PIPES,
TEST THERMOSTAT OPERATION,
INSPECT RADIATOR COOLING FAN(s)/CLUTCH OPERATION,
INSPECT CONDENSER/AUXILARY COOLING FAN(s) OPERATION(s)
INSPECT RADIATOR & CONDENSER FOR DEBRI,
OBSERVE COOLANT TEMPERATURE GAUGE OPERATION,

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Remove the firewall rubber grommet to the broken pipe in the firewall. Clean the remaining end of the broken pipe real good, apply a drop of oil on it, then push the heater hose back in and tighten the hose clamp back on. You will need a 12 point box end wrench and it will be slow due to limited access. It should work.

coolant will not flow from the return pipe continuously.when the engine heats the coolant expands returns to tank. coolant fan starts engine cools down return pipe flow stops . fan stops after times, again coolant heats up and flows through the pipe. this cycle continues and go on.

If your coolant level dropped you have a leak in the system. Just adding coolant will not solve the problem, as it will continue to overheat. You should have a pressure test done to determine where the leak is. To bleed the system you have to slide back the hose going onto the heater core. At the back of the engine you will see two hoses going thru the fire wall. The hose on the left has a hole in it. Loosen the clamp ,slide the hose back . Add coolant to the system until it flows freely from the hole.Push hose back onto pipe ,tighten the clamp. Start the engine and turn on heater controls to high and run engine until normal operating temp is reached. Stop engine and check coolant level in the expansion tank and add if necessary.

There is no radiator cap on the Northstar engine.
The only cap is on the surge tank (or expansion tank) and is pressurized.

So the answer is Yes, a faulty surge tank cap will result in the loss of coolant and therefore lead to overheating.

The most common overheating problem with the Northstar engine is a blocked purge line(small hose).The purge line is designed to purge air out of the cooling system.
There is a small diameter hose coming from near the top of the coolant tank (not the cap) ,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.
If the hose is clear, check for blockage at the nipple on the tank.Or trace the hose back until you find the blockage.
The hose goes in to the engine to a crossover and comes out the other side close to the thermostat housing,the blockage may be there.

The most common overheating problem with the Northstar engine is a blocked purge line. It's job is to purge air out of the cooling system.
There is a small diameter hose coming from near the top of the coolant tank(not the cap),this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.If the hose is clear, check for blockage at the nipple on the tank.Or trace the hose back until you find the blockage.The hose goes in to the engine to a crossover and comes out the other side to near the thermostat housing,the blockage may be there.

Its located on the brake caliper and goes up to the brake line,To change just remove the high pressure fitting from both ends [very basic job]Fit the new pipe ,but now you have a new pipe but full of air!.To bleed out the new pipe just open the bleed screw [looks like a grease nipple on the top of the brake caliper]and connect a small piece of hose [must be a tight fit on the bleed screw]with the bleed screw loose and the hose fitted just fit the other end in to a container of brakefluid and press the brake slowly up and down till all the bubbles are gone{dont forget to top up your reservoir before you bleed it!!]as if the reservoir runs dry you will need a special bleeder to bleed the brakes [it requires 2 bar or 30 psi pressure to bleed out the system after the reservoir goes dry]

Check for coolant flow at the Purge line,this is a common problem on the Northstar.
There is a small diameter hose coming from near the top of the coolant tank,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.If the hose is clear, check for blockage at the nipple on the tank.

I am assuming you have the Northstar engine.

Check for coolant flow at the Purge line,this is a common problem on the Northstar.
There is a small diameter hose coming from near the top of the coolant tank,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.If the hose is clear, check for blockage at the nipple on the tank.

Are you running it without a thermostat? If so,then strangely enough,this will cause overheating as the Northstar cooling system is different to most conventional systems and therefore will not help you diagnose the problem.

head gasket